We will develop techniques for high resolution labeling of cell surface receptors and internal structures of platelets, using colloidal gold labeling in conjunction with correlative light and electron microscopy. VDIC LM, interferencelreflectance and AIC-LM tracldng of colloidal gold labeled receptors on living cells will be correlated with subsequent HRSEM. High resolution imaging of protein molecules labeled or unlabeled with small gold particles will be correlatively studied by SEM and aton-dc force microscope (AFM). We will develop preparative techniques involving high-pressure freezer, freeze-drying, freeze substitution, and critical point drying. Labeling is to be at the molecular and submolecular level of resolution such that active fragments and multiple epitopes on a single molecule can be identified and their spatial relationships seen and correlated with cell surface and internal ultrastructure.